Plot of log (3k ∫ T_RdV/8π³νSμ²) versus energy of the upper state in the transition (E_upper) for the transitions of ground (vibrational) state SiC₂ in Fig. 2. The data indicate T_rot1 = 30 K and T_rot2 = 130 K for the excitation temperatures within and across K‐ladders, respectively. The column density of SiC₂ is N(ν₃ = 0) 2.2 × 10¹⁵ cm⁻². If, on the other hand, we assume T_rot1 = 15 K and T_rot2 = 145 K (see text), we obtain N(ν₃ = 0) 1.1 × 10¹⁵ cm⁻².